In intensive care therapy, ventilators (also called respirators) are used for mechanical ventilation of the lungs of a patient. The ventilator unit is connected to a hose set; the ventilation tubing or tubing circuit, delivering the ventilation gas to the patient. At the patient end, the ventilation tubing is typically connected to a tracheal ventilation catheter or tube, granting direct and secure access to the lower airways of a patient. Heat and moisture exchange (HME) devices are used as part of the breathing tube system for patients requiring breathing assistance or air supply. They are typically located at the patient end of the tubing, adjacent the tracheal catheter.
In normal, unassisted respiration, heat and moisture are absorbed from the exhaled air by the inner walls of the oral and nasal cavities and the pharynx as it travels from the lungs to the outside environment. This heat and moisture is then transferred to the inhaled air in the next breath, helping to keep the mucus membranes of the lungs humidified and at the proper temperature. Mechanical ventilation the lungs bypasses this natural system, resulting in warm, dry air being introduced to the lungs. After a period of time, the respiratory tract of a ventilated patient becomes dried, causing pain and discomfort and possibly causing lung damage.
HME devices are placed in the ventilator circuit at a point where the warm, moist air leaves the patient. The exhaled air immediately enters the device, where the moisture and heat are absorbed by materials disposed in the path of the flow of air. These layers then impart the absorbed heat and moisture to the inhaled air in the next breath. The retention of warmth and the high humidity prevent the patient's lungs and mucus layers from drying out. In addition, HME devices can act as a viral filters to prevent viruses from entering the breathing air supply of the patient.
While warmth and humidity are desired in the lungs, liquid moisture is not. Liquid moisture prevention is usually provided by specialized foam in the HME device which functions to trap any liquid moisture from entering the lungs. With time, however, moisture can build up in the foam causing an increase in the pressure drop across the device. This greater pressure drop increases the effort that must be expended by the patient and can cause the patient to labor to breathe. At present, the care giver observes the labored breathing of the patient and uses this as the signal to change out the HME device.
Rather than require each patient to fatigue himself by laboring to breathe, It would clearly be useful to have an indicator of some sort built into the device to alert the care-giver that the HME device has a certain level of moisture and needs to be changed.